The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Plasma etching is frequently used in semiconductor fabrication. In plasma etching, ions are accelerated by an electric field to etch exposed surfaces on a substrate. The electric field is generated based on RF power signals generated by a radio frequency (RF) generator of a RF power system. The RF power signals generated by the RF generator must be precisely controlled to effectively execute plasma etching.
A RF power system may include a RF generator or supply, a matching or match network, and a load (e.g., a plasma chamber). The RF generator generates RF power signals, which are received at the matching network. The matching network matches an input impedance of the matching network to a characteristic impedance of a transmission line between the RF generator and the matching network. This impedance matching aids in maximizing an amount of power forwarded to the matching network (“forward power”) and minimizing an amount of power reflected back from the matching network to the RF generator (“reverse power”). Forward power may be maximized and reverse power may be minimized when the input impedance of the matching network matches the characteristic impedance of the transmission line.
In the RF power generator or supply field, there are typically two approaches to applying the RF signal to the load. A first, more traditional approach is to apply a continuous wave signal to the load. In a continuous wave mode, the continuous wave signal is typically a sinusoidal wave that is output continuously by the power source to the load. In the continuous wave approach, the RF signal may be a sinusoidal output, and the amplitude and/or frequency of the sinusoidal wave can be varied in order to vary the output power applied to the load.
A second approach to applying the RF signal to the load involves pulsing the RF signal, rather than applying a continuous wave signal to the load. In a pulse mode of operation, a RF sinusoidal signal is modulated by a modulation signal in order to define an envelope for the modulated sinusoidal signal. In a conventional pulse modulation scheme, the RF sinusoidal signal typically is output at a predetermined frequency and amplitude. The frequency can be varied to improve impedance match conditions, providing agile frequency tuning. Amplitude may be varied to change the power of the RF signal. Power delivered to the load may also be controlled by varying the modulation signal, in addition to or rather than varying the sinusoidal, RF signal.
In a typical RF power generator configuration, output power applied to the load is determined using sensors that measure the forward and reflected power or the voltage and current of the RF signal applied to the load. Either set of these signals is analyzed to determine the parameters or electrical characteristics of the power applied to the load. The parameters can include, for example, voltage, current, frequency, and phase. The analysis may determine a power value which is used to adjust the output of the RF power supply in order to vary the power applied to the load. In a RF power delivery system, where the load is a plasma chamber, the varying impedance of the load causes a corresponding varying power applied to the load, as applied power is partially a function of the impedance of the load. Therefore, the varying impedance can necessitate varying the parameters of the power applied to the load in order to maintain optimum application of power from the RF power supply to the load.
In plasma systems, power is typically delivered in one of two configurations. In a first configuration, the power is capacitively coupled to the plasma chamber. Such systems are referred to as capacitively coupled plasma (CCP) systems. In a second configuration, the power is inductively coupled to the plasma chamber. Such systems are typically referred to as inductively coupled plasma (ICP) systems. Plasma delivery systems typically include a bias and a source that apply respective bias power and source power to one or a plurality of electrodes. The source power typically generates a plasma within the plasma chamber, and the bias power tunes the plasma to an energy relative to the bias RF power supply. The bias and the source may share the same electrode or may use separate electrodes, in accordance with various design considerations.
When a RF power delivery system drives a load in the form of a plasma chamber, the electric field generated by the power delivered to the plasma chamber results in ion energy within the chamber. One characteristic measure of ion energy is the ion energy distribution function (IEDF). The ion energy distribution function (IEDF) can be controlled with a RF waveform. One way of controlling the IEDF for a system in which multiple RF power signals are applied to the load occurs by varying multiple RF signals that are related by frequency and phase. The frequencies between the multiple RF power signals are locked, and the relative phase between the multiple RF signals is also locked. Examples of such systems can be found with reference to U.S. Pat. Nos. 7,602,127; 8,110,991; 8,395,322; and 9,336,995 assigned to the assignee of the present invention and incorporated by reference in this application.
RF plasma processing systems include components for plasma generation and control. One such component is referred to as a plasma chamber or reactor. A typical plasma chamber or reactor utilized in RF plasma processing systems, such as by way of example, for thin-film manufacturing, utilizes a dual frequency system. One frequency (the source) of the dual frequency system controls the generation of the plasma, and the other frequency (the bias) of the dual frequency system controls ion energy. Examples of dual frequency systems include systems that are described in U.S. Pat. Nos. 7,602,127; 8,110,991; 8,395,322; and 9,336,995 referenced above. The dual frequency systems described in the above-referenced patents include a closed-loop control system to adapt RF power supply operation for the purpose of controlling ion density and its corresponding IEDF.
The demand on plasma processing accuracy continues to increase. Tighter tolerances are being required of plasma-based fabrication systems, including decreasing component size, increasing density, both of which require greater accuracy from the plasma-based fabrication processes. Further challenges exist in connection with three dimensional integrated circuit and memory fabrication processes. One approach to significantly increasing the density of memory components is to fabricate memory components in a three dimensional structure. Three dimensional etching requires tight tolerances to direct ions to carry out the fabrication process. Some three dimensional etching processes require a 40:1 or greater aspect ratio. That is, the channel holes etched can be at least forty times taller than wide. In order to properly etch to these tolerances, it is necessary to direct ions at the wafer under fabrication in a substantially orthogonal direction, or directly at the workpiece wafer under fabrication, to provide sufficient yields. Other applications that require similarly accurate directivity of the ions at a substantially orthogonal direction to the wafer include solar or flat panel display fabrication and multiple electrode plasma fabrication systems.
Further complicating the control of plasma-based fabrication process is that the distribution of electrical power across the surface of a wafer may not be uniform. The electric field or electrical power near the edges of a workpiece or wafer may vary relative to the electrical power or fields away from the edges of the wafer. This variation can cause the ions to move in a direction less orthogonal to, or more across, the wafer, thereby making it difficult to meet the tolerances required for efficient fabrication, such as for three dimensional structures. One approach to improving the directivity of the ions near the edge of the wafer places a secondary electrode, sometimes referred to as an auxiliary electrode, near the edge of the wafer to provide a supplemental electrical field near the edge of the wafer. The secondary electrode can be powered independently by a separate RF generator and enables the tuning of the electrical power and field near the edge of the wafer, thereby enabling an increased control of the angle of incidence of the ions upon the wafer.
Present methods of providing RF power to the auxiliary electrode include passive reactive termination of the auxiliary electrode, such as with a variable capacitor. Other methods include using a slave or secondary RF generator operating in phase lock loop with respect to a master or primary RF generator. In a pulsed implementation, however, these methods may not provide the desired directivity of the ions in the plasma based fabrication system.